With packet communication carried out in, for example, the Internet, when encoded information cannot be received at a decoding apparatus due to, for example, loss of packets in the transmission path, processing to repair (conceal) the loss of these packets is typically carried out.
For example, in the field of speech encoding, in the ITU-T recommendation G.729, frame erasure concealment processing is defined where: (1) a synthesis filter coefficient is repeatedly used; (2) pitch gain and fixed codebook gain (FCB gain) are gradually attenuated; (3) an internal state of an FCB gain predictor is gradually attenuated; and (4) a excitation signal is generated using one of an adaptive codebook or a fixed codebook based on determination results of a voiced mode/unvoiced mode in an immediately preceding normal frame (for example, refer to patent document 1).
In this method, voiced mode/unvoiced mode is determined using the magnitude of pitch prediction gain using pitch analysis results carried out at a post filter, and, for example, when a immediately preceding normal frame is a voiced frame, a excitation vector for a synthesis filter is generated using an adaptive codebook. An ACB (adaptive codebook) vector is generated from an adaptive codebook based on pitch lag generated for frame erasure concealment processing use, and this is multiplied with pitch gain generated for the frame erasure concealment processing use and becomes an excitation vector. Decoding pitch lag used immediately before is incremented and is used as the pitch lag for the frame erasure concealment processing use. The decoding pitch gain used immediately before is attenuated by a constant number of times and is used as the pitch gain for the frame erasure concealment processing use.    Patent Document 1: Japanese Patent Application Laid-open No. Hei.9-120298.